The applicant proposes to analyze the factors which limit calcium filling by cardiac sarcoplasmic reticulum. The factor to be principally investigated is Ca-Ca exchange. The approach is to analyze the effects of calcium, magnesium, K ion ATP0and ADP on ATPase activity, calcium capacity and calcium-calcium exchange. The applicant also proposes to establish conditions which will permit the labeling of the catalytic and effector sites with photoaffinity labels and then to determine the topological organization of these sites by degrading the enzyme with trypsin and other proteolytic enzymes.